The present invention relates to a method for noise reduction during FM reception by attenuating a detected audiosignal, a field strength indicative muting control signal being derived from an IF signal which is used to control an attenuation of the audiosignal, when the field strength decreases below a certain threshold level and a receiver for executing the method.
A receiver executing a noise reduction method is known e.g. from U.S. Pat. No. 5,410,751 and has an RF front end for tuning the receiver onto a wanted RF FM signal. The RF front end demodulates the received RF FM signal into an IF signal. The RF front end is subsequently coupled to a selective IF amplifier for selecting and amplifying the IF signal and an FM detector for detecting the IF signal to obtain its baseband FM modulation signal, which is an audiosignal. The baseband FM modulation signal is supplied to a muting circuit. The muting circuit suppresses FM demodulation noise under control of the field strength indicative muting control signal. The FM demodulation noise is indicated as FM interstation noise and occurs at the output of the FM detector. The muting control signal is derived from the IF signal by measuring the level thereof in an IF level detector coupled to the IF amplifier and filtered and/or smoothed in a lowpass filter following the IF level detector.
FM noise occurring in the baseband FM modulation signal at the output of the FM detector and also in the audio signals derived from the detector, increases inversely to the field strength of the received FM RF signal. In particular FM RF signals decreasing below a certain threshold value give rise to a sharp increase in FM noise. In the known receiver, this FM noise is suppressed by an attenuation or mute of the baseband FM modulation signal in the muting circuit every time the lowpass filtered IF signal level drops below a certain threshold value. However, the IF signal level is not always related one to one to the field strength of the received RF FM signal. Due to the passband frequency characteristic of the IF selectivity, the IF signal level not only varies with the field strength of the received RF FM signal, but also with the deviation of the momentary frequency of the IF FM signal from the IF carrier frequency. Depending on the magnitude of deviation, filter side suppression may occur, reducing the IF signal level considerably. This may occur in the absence of FM noise and even when receiving undisturbed RF FM signals of moderate strength. Such frequency deviation dependent reductions in the IF signal level, in particular when occurring with FM RF signals received with low field strength, may give rise to unwanted mute actions and signal attenuations, resulting in distortions of the useful audiosignal.
In the known receiver the lowpass filter following the IF level detector at the output of the IF amplifier smooths the muting behavior and introduces a time constant in the mute control, thereby delaying the mute control to some extent. The choice of this time constant is critical with regard to the balance between signal distortion on the one hand and audible FM noise on the other hand. The lower the cutoff frequency of the lowpass filter, the larger the time constant and delay in the mute control, the less mute actions and the less distortions will occur, but this also results in the less alert FM noise being suppressed thereby causing FM noise to be increasingly audible, and vice versa.
Thus there is a need to introduce an additional degree of freedom in the mute control providing a solution to the above conflicting requirements to the time constant of the mute control.
A method of field strength dependent noise reduction during FM reception as mentioned here above according to the invention is therefore characterized by a time constant in the mute control being increased with an increasing deviation of the momentary frequency of the received FM signal from the FM carrier frequency and vice versa.
The method according to the invention differentiates in its mute responsiveness to reductions in the IF signal level at the output of the IF selectivity between reductions due to filter side suppression and those due to decreasing field strength. It is based on the recognition that IF signal level reductions due to filterside suppressions are mainly caused by undisturbed, low modulation frequencies and do not need to effect any muting. IF signal level reductions caused otherwise result mainly from decreasing field strength giving rise to audible FM noise, which need to be muted.
By applying this measure according to the invention, the time constant of the lowpass filter is decreased to a small value, thereby making the mute control highly responsive to IF level reductions, when the modulation frequency is varying within the pass band of the IF selectivity. This results in an alert and adequate muting of FM noise. In the following muting is to be understood as any decrease in signal gain or increase in signal attenuation, be it stepwise or gradual. The time constant of the lowpass filter and that of the mute control is considerably increased when the momentary frequency of the IF signal deviates from the carrier frequency to such extent that due to filter side suppression the IF signal level is noticeably reduced. This prevents unnecessary mute actions and distortions of the useful audio signals from occurring.
Preferably a time constant control signal reflecting the deviation of the momentary frequency of the IF signal from the IF carrier frequency is derived from the level of detected baseband FM modulation signal being obtained by means of a rectifier having lowpass filter functionality for rectifying and selecting the detected baseband FM modulation signal being supplied from the output of the FM detector to the rectifier.
In another preferred embodiment of the invention, a non-linear amplifier is used to amplify the time constant control signal with a non-linear gain allowing more accurate adjustment of the time constant variation of the mute control to the occurrence of FM noise as a function of the deviation of the momentary frequency of the IF signal from the IF carrier frequency.
In yet another preferred embodiment of such a receiver using analog circuitry the FM modulation signal level detector is subsequently coupled between the output of the FM detector and the time constant control terminal of the lowpass filter, a DC blocking filter and the rectifier. The DC blocking filter eliminates any DC component in the baseband FM modulation signal which may result e.g. from tolerance differences in the circuitry elements of in particular the IF amplifier. In the receiver shown, the rectifier is provided with a post detection lowpass filter with averaging function.
It is to be understood that both the foregoing general description and the following detailed description are not limiting but are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention.